Post on 14-Jan-2022
Institute for Nuclear and Particle Physics - TU Dresden
Abhishek Nag
Search for resonant WZ production at the LHC
July 8, 2021
July 8, 2021
Introduction: Standard Model
The Standard Model does not adequately explain:
● Gravity● Dark Matter● Dark energy● Neutrino Masses● Matter- antimatter asymmetry
Problems:
● Hierarchy problem → couldn’t explain the low mass of Higgs
● CP violation → weak interactions violate not only the charge-conjugation symmetry C between particles and antiparticles and the P or parity, but also their combination.
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● Need beyond SM physics to explain such problems.
July 8, 2021
Large Hadron Collider
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Accelerates proton bunches on 27 km ring and collides them at √s = 13 TeV
July 8, 2021
ATLAS DetectorElectrons:
● tracks in Tracker● Energy deposits in EC
Quarks & Gluons (as jets):
● tracks in Tracker● Showers in EC + HC
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Muons:
● tracks in Tracker & MS● Energy deposits in EC + HC
Neutrinos: reconstructed missing transverse energy (ET
miss)
The goal is a precise measurement of the charge, position and energy of the particle
July 8, 2021
Resonances
● If (pA+pB)2≈ m2C, amplitude 𝑴 and consequently the cross
section becomes large → resonances○ extremely short lived
● In the past, resonances have been helpful in discoveries, e.g. Z boson
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s-channel resonance diagram Amplitude M
Invariant mass of A+B: s = (pA + pB)2
[1]
July 8, 2021
How to look for new physics?
● Direct Search○ Enough energy at LHC to produce the particle on shell.○ Looking for a bump in the invariant mass distribution.
● Indirect Search○ New particles produced at much higher energy than that of LHC.○ Deviations from the SM observables like cross-sections, mass,
couplings showing up in precision measurements■ Effective Field theory approach
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New physics may be beyond our reach
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Vector Boson pairs
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Vector Boson pair production
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New physics in tail
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Effective Field Theory (EFT)
● An EFT is a low-energy approximation for a more fundamental theory involving interactions at a mass scale Λ
● Deviations are parameterized by higher order operators from the SM fields
● dim-6 terms are generally better constrained in Triple Gauge Couplings --> Look at dim-8 for Vector Boson Scattering
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operators of odd dimension violate baryon or lepton number4
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EFT example
● If tails show discrepancies with the SM → can be used to look for new physics
● Recent CMS result published [3]
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[2]
WZjj → lvlljj from CMS
cj/Λ4
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New physics in bump
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● Resonance searches: the simplest way to discover new particles.● Observe a statistically significant bump above a smooth background● Model-independent probe to new physics.
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Fig: Feynman diagram for W’ -> WZ process at the LHC
● Resonance benchmark:○ Heavy vector Triplets (simplified Lagrangian) produced either by qqF or VBF○ Georgi - Machacek (GM) Higgs Triplet Model produced via VBF
Resonance Search: Looking for a bump
July 8, 2021
GM Model
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● GM model[4] is minimal extension of the SM Higgs sector
● Extended by two isospin triplets: one complex and one real scalar
● A custodial SU(2) symmetry is imposed.● 10 physical Higgs scalars: a fiveplet (H5), a triplet (H3)
and two singlets (h, H).● The fiveplet H5 is fermiophobic and couples to vector
bosons.● Use H5Plane benchmark[5]
○ The mass of H3 is larger than the mass of H5
Cheng-Wei Chang, CEPC 2017
July 8, 2021
Heavy Vector Triplets
● Parameterized Lagrangians[6] incorporating heavy vector triplet (HVT) permit interpretation of searches for vector resonances.
● Simplified phenomenological Lagrangian used.● Two benchmark models used:
○ Model A: heavy vectors emerging from an underlying weakly-coupled extensions of the SM gauge group (gv =1)
○ Model B: strongly coupled Composite Higgs scenario (gv =3)
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July 8, 2021
● We look into fully leptonic decay mode:○ Small branching fractions○ Clean signature, low background○ Good sensitivity to low mass
● Experimental signature:○ 3 hight pT isolated leptons○ Missing transverse energy (ET
miss)
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Selected events are separated into two categories targeting 2 production modes:
(a) qq̄ fusion (b) vector boson fusion
VBF category is defined exploiting the typical VBF topology:● two ”tagging” jets in forward region in opposite
direction● high invariant mass of tagging jets mjj.● vector bosons between tagging jets
WZ Resonance search analysis strategy
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WZ inclusive Selection
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Designed to select good W and Z pairs decaying leptonically
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Signal Optimization
● Improve sensitivity to resonant signals with additional selections● Assuming the resonant particle is heavy and at rest
○ W and Z are produced back to back○ Decay products of W and Z will be boosted → boost proportional to mass of resonant particle
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July 8, 2021
Signal Optimization
● 𝚫y(W,Z) < 1.5 → remove SM background● 𝚫φ(l, ET
miss) used to define 2 regions:○ Low mass : 𝚫φ(l, ET
miss) >1.5○ High mass: 𝚫φ(l, ET
miss) <1.5
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arXiv:1406.4456
July 8, 2021
Signal Optimization
● 𝚫y(W,Z) < 1.5 → remove SM background● 𝚫φ(l, ET
miss) used to define 2 regions:○ Low mass : 𝚫φ(l, ET
miss) <1.5○ High mass: 𝚫φ(l, ET
miss) >1.5
● LT the scalar sum of the leptons pT○ Cut value depending on mWZ
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arXiv:1407.3476
July 8, 2021
Signal Optimization
● 𝚫y(W,Z) < 1.5 → remove SM background● 𝚫φ(l, ET
miss) used to define 2 regions:○ Low mass : 𝚫φ(l, ET
miss) <1.5○ High mass: 𝚫φ(l, ET
miss) >1.5
● LT the scalar sum of the leptons pT○ Cut value depending on mWZ
● The ratio between the boson pT and the mWZ mass
○ pTW/mWZ and pT
Z/mWZ
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arXiv:1806.01532
July 8, 2021
Signal Optimization
● 𝚫y(W,Z) < 1.5 → remove SM background● 𝚫φ(l, ET
miss) used to define 2 regions:○ Low mass : 𝚫φ(l, ET
miss) <1.5○ High mass: 𝚫φ(l, ET
miss) >1.5
● LT the scalar sum of the leptons pT○ Cut value depending on mWZ
● The ratio between the boson pT and the mWZ mass○ pT
W/mWZ and pTZ/mWZ
● Other variables: Mjj, 𝚫yjj, 𝚫φjj, Centrality → for VBF like events which have 2 jets
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Signal Region definitions
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● Starts from the inclusive WZ selection● Events are separated into two categories → qq fusion and vector boson fusion● Two possibilities of analysis under study:
1. Cut based analysis: ■ Two orthogonal SR used to evaluate the HVT qqF, HVT VBF and GM VBF models
2. GM Neural Network analysis:
■ Deep Neural Network trained using events at the inclusive selection level + Mjj>100 GeV + Njet>=2■ Training aiming to discriminate GM signals from backgrounds■ GM VBF signal region defined with a cut on the DNN output
VBF signal region:● At least two VBF jets, choosing the 2
pt-leading ones as candidates● mjj>500 GeV and |∆Yjj|> 3.5
qqF signal region:● pZ
T /mWZ > 0.35 and pWT /mWZ > 0.35
● Fail VBF signal selection
July 8, 2021
1. SM WZ background:○ Dominant background○ Shape are estimated from theory ○ Normalization fitted in dedicated control regions
(orthogonal to SR)i. WZ VBF control region ii. WZ qq̄ control region
Background overview
1.2. Fake/Non prompt background:
○ Z+jets, Zγ, Wγ, ttbar, single top or WW → where jets or photons were misidentified as leptons○ Estimated using data
3. Other background○ ZZ production: normalization fitted in corresponding 4 lepton control region (ZZCR). ○ Others including ttbarV and triple boson production: Modeled using theory.
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WZ invariant mass
VBF SR
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Control Regions for prompt background estimation
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WZ VBF control region● At least two VBF jets, choosing the 2 pt-leading ones
as candidates● Low mjj or low |∆Yjj| → orthogonal to VBF SR● Fail qq signal region
Need to check if the theory prediction matches well with data
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Other Control regions
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WZ qqF control region ZZ control region
Let’s look at the invariant mass
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WZ Invariant mass distribution
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qq region vbf region
arXiv:1806.01532
July 8, 2021
Published Limits
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● Expected limits: fit model with background only hypothesis● Observed limits: fit data with model
arXiv:1806.01532
qq region vbf region
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Expected full RunII limits: qq Signal Region
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published 36 fb-1 limits
● Limit on model A improved from ~2.2 to ~2.6 TeV
Work in Progress
July 8, 2021
Expected full RunII limits: HVT VBF Signal Region
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published 36 fb-1 limits
Work in Progress
● Improvement in the limits
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Expected full RunII limits: GM VBF Signal Region
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published 36 fb-1 limits
Work in Progress
● Improvement in the limits
July 8, 2021
Combined LimitsIn different decay channels
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● Fully leptonic limits are better for low mass
● Limits with hadronic decay channel are better at high mass
arxiv
July 8, 2021
Take home message
● Resonances offer an easy way to look for new physics● Leptonically decaying W and Z bosons give low statistics but cleaner signature● Fully leptonic decay channel offer best limits for low mass resonances● We see improvements with the expected full RunII limits
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We are actively looking for physics beyond SM and there is room for surprises
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Thank you!
BACKUP
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July 8, 2021
Recent publication from CMS (link):
● Cut based analysis with additional cuts:
○ τh veto: hadronic decay of τ with pT > 18 GeV & |η|<2.3
○ max(Z*l)<1.0, where Z*
l is Zeppenfeld variable defined as
|ηl -(ηj1 + ηj2)/2|/|Δηjj|
● 2 dimensional fit with 7 bins in mT_WZ and 2 bins in Mjj
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linkCMS full RunII limits